Losing appetite

[Petite]

I'm a bit concerned about my dwarf, Enigma. He's around a year old, and for the last few weeks his appetite just hasn't been as it usually is.

I'm feeding him on Harry Hamster, and every second day I dispose of any uneaten food and leave fresh. The next day I top it up - except that often it needs very little topping up. I also give him some fresh veggies most days (cauliflower, broccoli, etc). He is keen enough when he sees these, but then has a brief nibble and leaves them, sometimes coming back later, sometimes not. He hasn't had any food that he is unaccustomed to recently. As treats I give him a couple of sunflower seeds each day, and he gets occasional millet seeds or a whimzee.

I don't generally weigh him, but I can tell by looser skin around his middle that he seems to have lost weight. I also think that he is a little less active, and more docile than usual. Is this natural ageing, or should I be worried?